The present invention relates to improvements to water-containing explosives. More particularly, the invention relates to salt phase sensitization (with nitroalkanes and mixtures of nitroalkanes and arenes) of such explosives to provide highly efficient explosives.
Water-containing explosives, such as emulsions, are based on water-in-oil dispersions having a discontinuous water phase, including oxygen supplying salts dissolved in water, dispersed in a continuous oil phase including light and heavy oils and emulsifying aids and agents. Emulsification under low to high shear process stabilizes the product. The predominant oxygen supplying salt is ammonium nitrate, although sodium nitrate and calcium nitrate, as well as mixtures of these nitrate salts, are frequently used. Other water containing explosives include slurries and water gels. Water gels are characterized by the presence of gums (e.g. galactomannan gums), thickeners, acids and cross-linking agents to provide a stable product. These water containing explosives typically contain in excess of 5% by weight of water and may contain up to about 20% by weight or more of water. Typically about 7 to about 17% by weight is present.
The presence of water reduces the available thermochemical energy provided by the dissolved salts and their fuels. A discrete salt phase (which will not substantially dissolve in the water phase) frequently is blended into an intermediate or final mixture to increase the total available thermochemical energy. This salt phase may also carry entrained air and thus reduce mixture density and add so-called "hot spots" which improve detonation sensitiveness. These salt phase-supplemented water-containing explosives may be known as "heavy ANFO."
ANFO, heavy ANFO, and other water-containing explosives are "non-ideal" explosives. Non-ideal explosives are products whose detonation and explosion state efficacies are relatively dependent upon their exterior "environment," and upon their criticality of diameter and density. By indusrry parlance the "environment" may include: (1) the structural nature of the rock to be blasted, (2) the type and degree of confinement of the product charged into the blast hole, (3) primer strength which will detonate the main charge, (4) blast geometry, shot balance and initiator delay firing pattern, (5) temperatures and humidity during product storage and during shot loading, (6) blast hole waterhead pressure, and (7) the effect of transient pressures from the adjacent firing holes.
"Ideal" explosives, on the other hand, tend to perform independently of their exterior environment. Examples include nitrogelycerin, PETN, RDX and TNT; these are well known high explosives which are frequently labelled as "molecular explosives."
It is well known by those skilled in the art and science of explosives that during the detonation state and explosion state reactions the maximum theoretical energy values of a mixture seldom, if ever, are reached, but may become more fully available when certain enhancing agents have been added to the formulation. Such agents by common parlance have been called sensitizers, energy enhancers, fuel boosters, etc. For purposes of this application these terms are combined into a single phrase to better describe their true function and contribution--Sensitizing Energy Release Agent (SERA).
Currently popular SERAs for example include but are not limited to the following groups: (1) molecular explosives, (2) aluminum granules,flakes and powders, (3) certain energetic chemicals such as, but not limited to, amine nitrates, nitroparaffins and perchlorates, and (4) spherical particles of encapsulated air or other gas. Spherical particles ("microspheres") may be closed or open cell, and range in useful diameters for explosives from about 10 microns to about 350 microns. Generally, a shell midrange of about 40 to 100 microns is preferred. Shell materials of the closed cell microspheres are ceramic, glass or glass-like, phenolic, and polyethylene. Most open cell types are perlites. Particle or liquid displacement densities of the popular varieties vary from 0.03 g/cc for polyethylene to about 0.7 g/cc for aluminum silicates (ceramics). The term "hollow glass microspheres" is frequently applied to the ceramics, the glass-like spheres, and even to perlites.
Until recently, water-containing explosives (WCE) most frequently have been sensitized by (1) incorporating energetic chemicals as part of a host matrix or concentrate, (2) adding from about 0.3% to about 7% by bulk weight of hollow glass microspheres (HGM), or (3) adding about 0.5% to about 30% by weight of aluminum particles. Sometimes both HGM and aluminum are used.
HGM reduces host density from above its critical density to below its critical density. In so doing, HGM also provide or increase the number of "hot spots" necessary in non-ideal explosives for continuation of the detonation wave front. Aluminum particles beneficially add to the heats of detonation and explosion, thus increasing resultant pressures to better fracture and displace the material being blasted.
It is known that superior blasting efficacy can be obtained with blasting agents made of 87-82% comminuted ammonium nitrate prills (AN), fueled and sensitized with 13-17% of 2-3 carbon nitroalkanes. Mixtures of 13% nitropropane/87% AN or 171/2% nitroethane/821/2% AN are oxygen balanced to near zero. These blasting agents are considerably more energetic than the ANFO compositions they may replace.
Those skilled in the art also know that low viscosity long chain hydrocarbons, e.g., No. 2 diesel fuel (fuel oil or FO), can economically replace the pure fuel contribution of the nitroalkane utilized as described. Nitropropane isoxygen deficient (negative) by 135 gram-atoms per 100 grams whereas fuel oil is generally recognized as negative 346 gram atoms per 100 grams. Thus for considerations of oxygen balance alone each weight percent of fuel oil can replace 2.56 weight percent of nitrogpropane. This trade-off in favor of fuel oil, for reason of economy, is at the expense of otherwise available energy enhancement.
It is known from Edwards et al., U.S. Pat. No. 4,273,049, that a satisfactory bulk blasting agent is achieved with a mixture of about 90% ammonium nitrate, about 7% nitropropane and about 3% fuel oil. This type of bulk blasting agent has no water resistance, however, and must be utilized in dry blast holes or with flexible plastic liners in dewatered blast holes. Also, since it contains no thickening agent it must be mixed and used reasonably promptly before the fuel oil and nitroalkane migrate away from the ammonium nitrate thus reducing sensitiveness.